OBJECTIVE: Research was undertaken to test two hypotheses. First, during the early neonatal period, thyroid function of very low birth weight (VLBW) infants is suppressed by exposure to iodine-containing antiseptic solutions and/or iodized contrast media. Second, this suppression is more pronounced in small for gestational age (SGA) infants. METHODS: Urinary iodine concentration and thyroid function measurements were obtained prospectively from 44 VLBW infants with gestational ages at birth of 30 +/- 2.3 weeks and weights of 1223 +/- 231 g. Eleven of these infants were SGA. The infants were grouped according to iodine exposure: 18 infants had no increased exposure and served as control infants; 9 infants were exposed to an iodine-containing antiseptic (povidone iodine); 12 infants were exposed to an iodized contrast medium (iopamidol); and 5 infants were exposed to both agents. Urinary iodine and serum free triiodothyronine, free thyroxine, and thyrotropin were measured on days 1, 7, 14, 21, and 28 of life. RESULTS: During the period of maximum exposure (days 1 to 7), the concentration of iodine in the urine of study infants was 2 to 4 orders of magnitude greater than that in the urine of control infants (123 +/- 141 micrograms/L). During the subsequent 3 weeks, levels of urinary iodine in study infants returned to levels that were not significantly different from controls. On day 7 of life, iodine-exposed infants had a significantly higher mean thyrotropin level than control infants, whereas on day 28, free triiodothyronine and thyroxine levels were lower. Of the 26 iodine-exposed infants, 6 had transient hyperthyrotropinemia and 2 had transient hypothyroidism. When exposed to iodine, SGA infants had more labile thyroid function than normally grown iodine-exposed or control infants. These SGA infants had significantly lower levels of thyroid hormones in umbilical cord blood, increased production of thyroid hormones on day 14 of life, and lower levels again at 1 month. CONCLUSION: In VLBW infants, the use of iodine-containing antiseptic solutions and iodized contrast media results in massive uptake of iodine that is associated with alterations in thyroid function. It is reasonable to suggest that, whenever possible, iodized products should be avoided in VLBW infants, because their routine use results in exposure to excessive loads of iodine, which can be associated with hyperthyrotropinemia and hypothyroidism.